Many prior workers have sought to increase the sorbency of fibrous web products by addition of "super absorbent" particles, e.g., modified starch or other polymeric particles which sorb and retain under pressure large volumes of liquids, especially aqueous liquids. The previous products prepared by such additions all have had significant limitations. For example, one commercial product, which comprises sorbent particles adhered between two sheets of tissue paper, decomposes in use, whereupon the sorbent particles are washed out of the product and into liquid being treated. Another commercial product, comprising a rather stiff open-mesh fabric or cheese cloth to which essentially a single layer of sorbent particles is adhered, sorbs only limited amounts of liquid.
A different product taught in U.S. Pat. No. 4,103,062 is made by dispersing particles in an air-laid cellulosic fiber web and densifying the web with heat and pressure to increase its strength. However, this product sorbs only a limited amount of liquid, because of the nonexpansible nature of the densified web, and because sorbent particles at the edge of the web swell upon initial liquid intake and prevent permeation of additional liquid into internal parts of the web. U.S. Pat. No. 4,105,033 seeks to avoid such edge blockage by distributing the sorbent particles in spaced layers separated by layers of fibers, but such a construction requires added processing steps and is subject to delamination. In other products sorbent particles are simply cascaded into a loose fibrous web (see U.S. Pat. No. 3,670,731), but both U.S. Pat. No. 4,103,062 and U.S. Pat. No. 4,105,033 note that it is difficult to deposit the particles uniformly, and the particles tend to move within the web during subsequent processing, storage, shipment or use of the web and thereby develop nonuniform properties.
U.S. Pat. No. 4,235,237 teaches a different approach in which a fibrous web is sprayed, immersed or otherwise contacted with sorbent material dispersed in a volatile liquid. Vaporization of the volatile liquid leaves a web in which sorbent particles envelop the fibers, principally at fiber intersections. Disadvantages of this approach include the need for multiple steps to prepare the product, limitations on amount of sorbent that can be added to the web, brittleness of the dried webs, and the tendency for sorbent material to be concentrated at the web surface.
Many of these problems have been overcome by the sorbent sheet product described in U.S. Pat. No. 4,429,001. In this product, an array of solid high-absorbency liquid-sorbent polymeric particles are uniformly dispersed within a coherent web of melt blown fibers. However, even greater improvement in the rate of liquid sorption would be desirable, since swelling of a mass of the high sorbency particles upon initial liquid sorption can still limit rapid permeation of additional liquid into internal parts of the web. Further, the coherency of the web of melt blown fibers tends to somewhat limit the swelling of the high-sorbency particles during liquid sorption. Surprisingly, the present invention provides a product having increased rate of sorption and greater liquid sorbency than the product disclosed in U.S. Pat. No. 4,429,001.